A partially dielectric-loaded waveguide supports modes with phase velocity νph>c/∈1/2, where ∈ is the dielectric constant of the lining. The coupling of a fast mode (with νph>c) with a slow mode (with c /∈1/2<νph<c) via a negative-energy beam mode gives rise to the explosive growth of all the three waves at the expense of the energy of the beam. The slow mode of lower frequency can be launched from an external source and can become an efficient wiggler for the generation of the fast free-electron laser mode